Downhole heat exchanger for a geothermal heat pump

ABSTRACT

The invention relates to a downhole heat exchanger for extracting geothermal energy from a borehole, wherein the inner surface of the exchanger tube comprises the following roughness values: a) an arithmetic mean roughness Ra according to DIN EN ISO 4287 in the range of 1 to 15 μm, b) an averaged roughness Rz according to DIN EN ISO 4287 in the range of 8 to 80 μm, and c) a maximum roughness depth Rz1 max according to DIN EN ISO 4287 in the range of 10 to 500 μm, comprising an improved precipitation film during operation, such that the entire surface of the exchanger tube is uniformly wetted.

The invention relates to a downhole heat exchanger for recovering geothermal energy from a borehole.

The recovery of geothermal energy from boreholes is carried out by extraction of thermal water from opened-up aquifers or by cooling of the earth along a borehole. Cooling of the earth is effected by means of various downhole heat exchangers. To extract heat from the earth, it is possible to use vaporizable refrigerants which recover the energy by boiling. Such direct boiling heat exchangers are being used to an increasing extent. Compared to brine heat exchangers, they offer a significantly higher degree of efficiency and in technical circles are considered to be the technology of the future. There are, for example, systems based on propane (R290), butane, ammonia (R717) or carbon dioxide (R744), with propane being preferred. A distinction is made between near-surface geothermal energy for direct utilization, for instance for heating and cooling, usually as heat pump heating, and deep geothermal energy for direct utilization in the heat energy method or indirectly for generation of electric power. Deep downhole heat exchangers with direct boilers are also referred to as heat pipes.

DE 42 11 576 Al and DE 298 24 676 U1 describe arrangements of heat pipes in which the heating zone of the heat pipe and thus the boiling of the liquid refrigerant are located in the lower part of the pipe. The vapor is generated by boiling of the liquid refrigerant; it is then conveyed upward in a pipe and releases its energy at the top by condensation. This is utilized directly or with the aid of a heat pump.

In WO 01/04550, the refrigerant is conveyed upward through a channel into the heat exchanger and through a second channel. Film vaporization is sought by means of a spiral track which has to be produced in a complicated manner. However, vaporization of the refrigerant over the entire length of the borehole and thus heat exchanger cannot be achieved using the arrangement described there, so that complete extraction of heat is not made possible.

The utility model DE 20 2004 018 559 U1 describes a heat generator for recovering geothermal energy from a borehole, in which a condensate stream distributor is incorporated in a heat exchanger pipe. Although wetting on all sides is likewise said to be achieved, film vaporization cannot be realized.

Finally, DE 10 2007 005 270 Al describes a downhole heat exchanger which contains a condensate stream distributor having condensate conveying devices arranged radially and/or tangentially to the wall of the heat exchanger pipe. A radially distributed condensate film is said to be produced in this way.

EP 1 450 142 A2 describes a heat exchanger pipe consisting of a filler-containing polymer material. The pipe serves to convey air as heat transfer medium.

Finally, WO 2008/113569 discloses a pipe arrangement for downhole heat exchangers, in which the pipes have at least one layer of a polymer molding composition which contains a filler or reinforcing material which increases the mechanical strength. Damage to the outer surface during installation and subsequent crack growth are said to be prevented in this way. The pipe arrangement is intended for transport of a liquid heat transfer medium.

It is an object of the invention to produce a complete falling film in a downhole heat exchanger by simple means, so that the entire interior surface of the heat exchanger pipe is uniformly wetted.

This object is achieved by a downhole heat exchanger designed as direct boiling heat exchanger for recovering geothermal energy from a borehole, in which the interior surface of the heat exchanger pipe has the following roughness parameters:

-   -   a) an arithmetic mean roughness Ra in accordance with DIN EN ISO         4287 in the range from 1 to 15 μm, preferably in the range from         2 to 12 μm and particularly preferably in the range from 3 to 7         μm,     -   b) an average peak-to-valley height Rz in accordance with DIN EN         ISO 4287 in the range from 8 to 80 μm, preferably in the range         from 10 to 60 μm and particularly preferably in the range from         15 to 40 μm, and     -   c) a maximum peak-to-valley height Rz1max in accordance with DIN         EN ISO 4287 in the range from 10 to 500 μm, preferably in the         range from 15 to 150 μm and particularly preferably in the range         from 25 to 65 μm.

The roughness measurement is carried out by the tracer method in accordance with DIN EN ISO 4288. In the roughness measurement using a mechanical tracer instrument, a tracer tip made of diamond is moved at constant speed over the surface of a specimen. The measurement profile is given by the vertical displacement of the tracer tip, which is generally measured by means of an inductive displacement measurement system. To describe a surface technically, standardized roughness parameters are obtained from the measured profile.

Ra is the arithmetic mean roughness from the absolute values of all profile values.

Rz is the average of the five peak-to-valley heights from the five individual measurements.

Rz1max is the greatest peak-to-valley height from the five individual measurements.

The downhole heat exchanger comprises a heat exchanger pipe which is connected to the earth via a packing material, for example bentonite. The vaporization of the refrigerant condensate occurs on the interior surface of the heat exchanger pipe. The upward transport of the vapor formed occurs in the center of the pipe.

The internal diameter of the heat exchanger pipe is generally in the range from 15 to 80 mm, preferably in the range from 20 to 55 mm and particularly preferably in the range from 26 mm to 32 mm.

The heat exchanger length is generally from 60 to 200 m, with greater or smaller lengths also being possible in individual cases. The heat exchanger is preferably from 80 to 120 m long.

As refrigerant, use is made of, for example, propane (R290), butane, ammonia (R717) or carbon dioxide (R744). Further suitable refrigerants are, for example, propene (R1270), tetrafluoroethane (R134a), difluoromethane (R32), pentafluoroethane (R125), a mixture of R32, R125 and R134a in a ratio of 23/25/52 (R407C) or a mixture of R32 and R125 in a ratio of 50:50 (R410A). According to physical laws, the interior of the heat exchanger is therefore under relatively high pressure. The refrigerant vapor which has ascended is compressed in a compressor and thus liquefied. Compression liberates heat of condensation which is discharged as useful heat. The cooled liquid refrigerant is fed via an expansion unit back to the heat exchanger and conveyed downward as falling film. The refrigerant here vaporizes again with uptake of the geothermal energy. As regards the details of the technical procedure, reference is made to the abovementioned prior art.

The heat exchanger pipe can, for example, consist of metal. In this case, the interior surface bears a rough coating. Of course, the exterior surface can also be coated here, for example for reasons of corrosion protection. The metal can be aluminum, an aluminum alloy, steel, for example stainless steel, or any other metal. Coating can be effected by powder coating or by coating with the melt of a further molding composition as described below, for example by means of extrusion coating.

However, the pipe preferably consists of plastic and particularly preferably of a thermoplastic molding composition. Such pipes can be rolled up so that it is not necessary to join comparatively short pieces to one another, e.g. by welding, during installation.

The molding composition used has to have sufficient stiffness for the wall thickness to be made thin for reasons of heat transfer. In addition, the plastic which forms the matrix of the molding composition has to be sufficiently resistant to the refrigerant and to the moisture in the earth. This means that the wall must not swell since this would be associated with undesirable length changes.

Suitable plastics are, for example, fluoropolymers such as PVDF, PTFE or ETFE, polyarylene ether ketones such as PEEK, polyolefins such as polyethylene or polypropylene and polyamides.

Among polyamides, particular preference is given to those whose monomer units contain an arithmetic mean of at least 8, at least 9 or at least 10 carbon atoms. The monomer units can be derived from lactams or w-aminocarboxylic acids. When the monomer units are derived from a combination of diamine and dicarboxylic acid, the arithmetic mean of the carbon atoms of diamine and dicarboxylic acid has to be at least 8, at least 9 or at least 10. Suitable polyamides are, for example: PA610 (which can be prepared from hexamethylenediamine [6 carbon atoms] and sebacic acid [10 carbon atoms], and the mean number of carbon atoms in the monomer units is thus 8), PA88 (which can be prepared from octamethylenediamine and 1.8-octanedioic acid), PA8 (which can be prepared from caprylic lactam), PA612, PA810, PA108, PA9, PA613, PA614, PA812, PA128, PA1010, PA10, PA814, PA148, PA1012, PA11, PA1014, PA1212 and PA12. The preparation of the polyamides is prior art.

Of course, it is also possible to use copolyamides based thereon, with monomers such as caprolactam also being used if desired.

It is likewise possible to use mixtures of various polyamides, provided the compatibility is sufficient. Compatible polyamide combinations are known to those skilled in the art; mention may here be made by way of example of the combinations of PA12/PA1012, PA12/PA1212, PA612/PA12, PA613/PA12, PA1014/PA12 and PA610/PA12 and also corresponding combinations with PA11. In the case of doubt, compatible combinations can be determined by means of routine tests.

The thermoplastic molding composition can be filled with reinforcing fibers and/or fillers. The fibers or filler particles which project at the surface in this way produce the required roughness. For this purpose, the molding composition contains from 0.1 to 50% by weight, preferably from 0.5 to 20% by weight and particularly preferably from 3 to 10% by weight, of fillers and/or fibers. In one embodiment, the molding composition contains only fibers. In another embodiment, the molding composition contains only fillers. In a further embodiment, the molding composition contains a mixture of fibers and fillers.

Suitable reinforcing fibers are, for example, glass fibers, basalt fibers, carbon fibers, aramid fibers and potassium titanate whiskers and also fibers composed of relatively high-melting polymers.

Suitable fillers are, for example, titanium dioxide, zinc sulfide, silicates, chalk, aluminum oxide and glass spheres.

The thermal conductivity of the heat exchanger walls can be increased by means of suitable reinforcing fibers or fillers. For this purpose, metal fibers can be used as fiber material or metal powders, carbon black, graphite, CNTs (carbon nanotubes), hexagonal boron nitride or combinations or mixtures of the various materials can be used as filler.

The molding composition can additionally contain the customary auxiliaries and additives, for example impact modifiers, plasticizers, stabilizers and/or processing aids.

In a further embodiment, the surface roughness is generated by compounding in a second polymer which is incompatible or only slightly compatible with the matrix polymer and is therefore dispersed only relatively coarsely. Suitable combinations of materials are, for example, polyamide/polypropylene and polyamide/ethylene-acrylic ester-acrylic acid copolymer/polypropylene.

The heat exchanger pipe can, in one embodiment, be made up of a single layer and thus consist of one of the above-described molding compositions over the entire wall thickness. In a further embodiment, the heat exchanger pipe is made up of a plurality of layers, with the inner layer consisting of one of the above-described molding compositions and the other layers having functions which are not performed sufficiently by the layer of molding composition having a rough surface, for example flexibility, impact toughness or barrier action toward the refrigerant or the moisture in the earth. If the layers do not adhere to one another sufficiently well, bonding agents can be used as described in the prior art.

Suitable layer sequences from the inside outward are, for example:

-   -   polyamide (for example PA12)/bonding agent/polypropylene or         polyethylene;     -   polyamide (for example PA12)/bonding agent/ethylene-vinyl         alcohol copolymer (EVOH)/bonding agent/polyamide;     -   polyamide/bonding agent/EVOH/bonding agent/polypropylene or         polyethylene;     -   polyamide/bonding agent/fluoropolymer (for example PVDF or         ETFE);     -   polyamide/adhesion-modified fluoropolymer;     -   polyamide/bonding agent/polybutylene-2,6-naphthalate/bonding         agent/polyamide.

Suitable bonding agents for the bonding of polyamide and polyolefins are, for example, polyolefins functionalized with maleic anhydride.

Polyamides such as PA12 and EVOH can, for example, be joined to one another with the aid of polyolefins functionalized with maleic acid or by means of polyamide blends corresponding to EP 1 216 826 A2.

Polyolefins functionalized with maleic acid, for example, are suitable as bonding agents for forming the bond between EVOH and polyolefins.

Bonding agents for joining polyamides and fluoropolymers are known, for example, from EP 0 618 390 A1, while adhesion-modified fluoropolymers can be prepared, for example, by mixing in small amounts of polyglutarimide as described in EP 0 637 511 A1, by functionalization with maleic anhydride or by incorporation of carbonate groups as described in EP 0 992 518 A1.

To support the effect of the surface roughness, the heat exchanger pipe can additionally contain internals as are known from the prior art, for example DE 10 2007 005 270 A1.

The invention results in the falling film having a uniform layer thickness over the circumference of the heat exchanger; streaming or separation of the film is prevented. Owing to the increased surface area, better heat exchange is made possible; at the same time, the flow velocity is decreased, which counters flooding of the lowermost part of the heat exchanger. 

1. A downhole heat exchanger, comprising a heat exchanger pipe, wherein an interior surface of the heat exchanger pipe has the following roughness values: a) an arithmetic mean roughness Ra in accordance with DIN EN ISO 4287 in the range from 1 to 15 μm; b) an average peak-to-valley height Rz in accordance with DIN EN ISO 4287 in the range from 8 to 80 μm; and c) a maximum peak-to-valley height Rz1max in accordance with DIN EN ISO 4287 in the range from 10 to 500 μm, wherein the roughness measurement is carried out in accordance with DIN EN ISO
 4288. 2. The downhole heat exchanger of claim 1, wherein: Ra is in the range from 2 to 12 μm; Rz is in the range from 10 to 60 μm; and Rz1max is in the range from 15 to 150 μm.
 3. The downhole heat exchanger of claim 1, wherein: Ra is in the range from 3 to 7 μm; Rz is in the range from 15 to 40 μm; and Rz1max is in the range from 25 to 65 μm.
 4. The downhole heat exchanger any of claim 1, wherein the heat exchanger pipe comprises a layer comprising a thermoplastic molding composition.
 5. The downhole heat exchanger of claim 4, wherein the heat exchanger pipe or an innermost layer of the heat exchanger pipe comprises a molding composition whose matrix comprises a fluoropolymer, a polyarylene ether ketone, a polyolefin, or a polyamide.
 6. The downhole heat exchanger of claim 4, wherein the heat exchanger pipe or an innermost layer of the heat exchanger pipe comprises a molding composition comprising from 0.1 to 50% by weight of at least one selected from the group consisting of reinforcing fibers and reinforcing fillers.
 7. The downhole heat exchanger of claim 1, wherein the heat exchanger pipe comprises a metal comprising a rough coating on an interior surface.
 8. A process for recovering geothermal energy from a borehole, the process comprising: vaporizing a vaporizable refrigerant in the downhole heat exchanger of calim 1; compressing the refrigerant vapor which has ascended in the heat exchanger pipe in a compressor, to liquefy the vapor and remove heat obtained from the heat of condensation; and then feeding the cooled liquid refrigerant back to the downhole heat exchanger as falling film which is conveyed downward.
 9. The downhole heat exchanger of claim 1, in the form of a direct boiling heat exchanger, which is suitable for recovering geothermal energy from a borehole.
 10. The downhole heat exchanger of claim 1, wherein the internal diameter of the heat exchanger pipe is in the range from 15 to 80 mm.
 11. The downhole heat exchanger of claim 1, wherein the internal diameter of the heat exchanger pipe is in the range from 20 to 55 mm.
 12. The downhole heat exchanger of claim 1, wherein the internal diameter of the heat exchanger pipe is in the range from 26 mm to 32 mm.
 13. The downhole heat exchanger of claim 1, having a length in a range from 60 to 200 m.
 14. The downhole heat exchanger of claim 1, having a length in a range from 80 to 120 m.
 15. The downhole heat exchanger of claim 6, wherein the molding composition comprises from 0.5 to 20% by weight of at least one selected from the group consisting of reinforcing fibers and reinforcing fillers.
 16. The downhole heat exchanger of claim 6, wherein the molding composition comprises from 3 to 10% by weight of at least one selected from the group consisting of reinforcing fibers and reinforcing fillers.
 17. The downhole heat exchanger of claim 6, wherein the molding composition further comprises at least one selected from the group consisting of an impact modifier, a plasticizer, a stabilizer, and a processing aid.
 18. The downhole heat exchanger of claim 5, wherein the heat exchanger pipe consist of a single layer comprising the molding composition.
 19. The downhole heat exchanger of claim 5, wherein the heat exchanger pipe comprises a plurality of layers.
 20. The downhole heat exchanger of claim 7, wherein the metal is selected from the group consisting of aluminum, an aluminum alloy, and steel. 